Spatial distribution and sampling size for monitoring of African maize stem borer, Busseola fusca (Fuller) (Lepidoptera: Noctuidae) on maize (Zea mays L.) in Southern Ethiopia

The African maize stem borer (Busseola fusca) is one of the important biotic constraints for maize production in sub-Saharan African. This study determined the spatial distribution and sampling sizes for African maize stem borer in Southern Ethiopia. Twenty four maize farms were visited in 12 localities at three growth stage of maize. Data were collected on the number of infested and not-infested plants, and the number of larvae and pupae. There were variations in the levels of infestations and population density of B. fusca in the study areas and years. Percent infestation at mid-whorl stage of maize ranged from 13.6 to 25.9% and 19.5 to 41.4% in 2015 and 2016, respectively. Infestation increased through time and at harvesting stage reached ranges of 36.8 to 68.8% in 2015 and 65.5 to 80.7% in 2016. The optimal sample size for four fixed precision levels of 0.10, 0.15, 0.20 and 0.25 were estimated with Iwao’s regression coefficients. The distribution pattern of B. fusca varied between maize growth stages, locations and years. At mid-whorl stage of maize, B. fusca infested plants were aggregated but in both at silking and harvesting stage uniformly distributed. At mid-whorl as well as silking stage of maize B. fusca larvae were aggregated. But, larvae at harvesting stage and pupae in both silking and harvesting stages of maize were randomly distributed. For 10% infestation, which is considered as action threshold level for stem borers management on maize, 22 sampling units (660 plants) at the precision of 20% are required.